1. Technical Field
The invention relates generally to reduced pressure fining, a process for removing trapped bubbles in molten material, e.g., molten glass. More specifically, the invention relates to a method for controlling the foam produced when the molten material encounters reduced pressure in a reduced pressure finer.
2. Background Art
In industrial glassmaking, a glass batch is made by mixing in blenders a variety of raw materials obtained from properly sized, cleaned, and treated materials that have been pre-analyzed for impurity. Recycled glass called cullet may also be mixed with the raw materials. For the most commonly produced soda-lime glass, these raw materials include silica (SiO2), soda (Na2O), lime (CaO), and various other chemical compounds. The soda serves as a flux to lower the temperature at which the silica melts, and the lime acts as a stabilizer for the silica. A typical soda-lime glass is composed of about seventy percent silica, fifteen percent soda, and nine percent lime, with much smaller amounts of the various other chemical compounds. The glass batch is conveyed to a “doghouse”, which is a hopper at the back of the melting chamber of a glass melting furnace. The glass batch may be lightly moistened to discourage segregation of the ingredients by vibrations of the conveyor system or may be pressed into pellets or briquettes to improve contact between the particles.
The glass batch is inserted into the melting chamber by mechanized shovels, screw conveyors, or blanket feeders. The heat required to melt the glass batch may be generated using natural gas, oil, or electricity. However, electric melting is by far the most energy efficient and clean method because it introduces the heat where needed and eliminates the problem of batch materials being carried away with the flue gases. To ensure that the composition of the molten glass is homogenous throughout, the molten glass is typically stirred together in a conditioning chamber that is equipped with mechanical mixers or nitrogen or air bubblers. The molten glass is then carried in a set of narrow channels, called forehearth, to the forming machines. In the melting chamber, large quantities of gas can be generated by the decomposition of the raw materials in the batch. These gases, together with trapped air, form bubbles in the molten glass. Large bubbles rise to the surface, but, especially as the glass becomes more viscous, small bubbles are trapped in the molten glass in such numbers that they threaten the quality of the final product. For products requiring high quality glass, e.g., optical lenses, television panels, and liquid crystal displays, the trapped bubbles are removed from the molten glass prior to feeding the molten glass into the forming machines.
The process of removing bubbles from molten glass is called fining. One method for fining glass involves adding various materials known as fining agents to the glass batch prior to mixing in the blenders. The primary purpose of the fining agents is to release gas in the molten glass when the molten glass is at the proper fining temperature. The released gas then diffuses into gas bubbles in the molten glass. As the bubbles become larger, their relative buoyancy increases, causing them to rise to the surface of the molten glass where they are released. The speed at which the bubbles move through the molten glass may be increased by reducing the viscosity of the molten glass, and the viscosity of the molten glass can be reduced by increasing the temperature of the molten glass. An effective fining agent for atmospheric pressure, glass melting and fining processes should be able to release a large amount of fining gases as the temperature of the molten glass is increased to the temperature range where the viscosity of the molten glass is sufficiently low, i.e., 1300° C. to 1500° C. for soda-lime glass. Examples of fining agents that are suitable for use with soda-lime glass are arsenic oxide (As2O3) and antimony oxide (Sb2O3). These fining agents are, however, detrimental to the environment and require careful handling.
Another method for fining glass involves passing the molten glass through a low pressure zone to cause the bubbles in the molten glass to expand and rise quickly to the surface of the glass. This process is typically referred to as reduced pressure fining or vacuum fining. There are various configurations of reduced pressure finers. U.S. Pat. No. 5,849,058 to Takeshita et al. discloses the general structure of a siphon-type reduced pressure finer. The reduced pressure finer, as shown in FIG. 1, includes a vacuum vessel 1 disposed in vacuum housing 2. The vacuum vessel 1 has one end connected to an uprising pipe 3 and another end connected to a downfalling pipe 4. The uprising pipe 3 and the downfalling pipe 4 are made of platinum, a material that can withstand the high temperature of the molten glass and that is not easily corroded. The vacuum vessel 1, the uprising pipe 3, and the downfalling pipe 4 are heated by electricity. An insulating material 5 is provided around the vacuum vessel 1, the uprising pipe 3, and the downfalling pipe 4. Typically, the insulating material 5 consists generally of insulating bricks and doubles as a structural support for the uprising pipe 3 and the downfalling pipe 4. The bottom ends of the uprising pipe 3 and the downfalling pipe 4 that are not connected to the vacuum vessel 1 extend through the vacuum housing 2 into the storage vessels 6 and 7, respectively. The storage vessel 1 is connected to receive molten glass from a glass melting furnace (not shown).
Flow of molten glass through the uprising pipe 3, the vacuum vessel 1, and the downfalling pipe 4 follows the siphon principle. Accordingly, the liquid surface of the molten glass in the vacuum vessel 1 is higher than the liquid surface of the molten glass in the storage vessel 6, and the pressure in the vacuum vessel 1 is lower than the pressure in the storage vessel 6. The pressure in the vacuum vessel 1 is related to the elevation of the liquid surface of the molten glass in the vacuum vessel 1 with respect to the liquid surface of the molten glass in the storage vessel 6. The height of the liquid surface of the molten glass in vacuum vessel 1 with respect to the liquid surface of the molten glass in the storage vessel 6 is set based on the desired fining pressure and the rate at which molten glass is flowing into the vacuum vessel 1. The molten glass with the trapped bubbles is transferred from the glass melting furnace (not shown) into the storage vessel 6. Because the pressure in the vacuum vessel 1 is less than the pressure in the storage vessel 6, the molten glass in the storage vessel 6 rises through the uprising pipe 3 into the vacuum vessel 1. The pressure in the vacuum vessel 1 is brought to reduced pressure condition of less than the atmospheric pressure, typically  1/20 to ⅓ atmospheric pressure. As the molten glass passes through the vacuum vessel 1 and encounters the reduced pressure, the bubbles in the molten glass expand and quickly rise to the surface of the molten glass. The refined glass descends into the storage vessel 7 through the downfalling pipe 4. 
Foam is produced in the headspace 8 as the molten glass encounters the reduced pressure in the vacuum vessel 1. The headspace 8 must either be large enough to contain the foam, or the foam must be controlled, to prevent equipment flooding and other process upsets and quality problems. In large scale processes, it is usually not practical to make the headspace 8 big enough to contain the foam, especially because the headspace 8 must be maintained airtight. U.S. Pat. No. 4,704,153 issued to Schwenninger et al. discloses a method for controlling foam that includes providing a burner in the headspace. Schwenninger et al. in the '153 patent disclose that the heat from the burner reduces the viscosity of the foam and increases the volume of the bubbles of the foam, causing the bubbles to burst. U.S. Pat. No. 4,794,860 issued to Welton discloses a foam control method that includes applying agents to the foam, which cause coalescence of the bubbles and/or interrupt the surface tension in the bubble membranes so that the bubbles burst. Examples of foam breaking agents include water, alkali metal compounds such as sodium hydroxide or sodium carbonate, alcohol, and fuel oil. U.S. Pat. No. 4,849,004 issued to Schwenninger et al. discloses a foam control method that includes suddenly changing the pressure in the headspace so as to disrupt the bubble membranes of the foam, thereby bursting a substantial portion of the bubbles and expediting collapse of the foam. A sudden surge of low pressure is used to expand the foam bubbles beyond their limit of elasticity, at which point they break. The pressure surges may be applied at intervals of several minutes, and the duration of the pressure surges may be on the order of a few seconds.